Microwave Dielectric Properties of Ca4(La4Pr2)(SiO4)4(PO4)2O2 Ceramics Doped with Isovalent and Aliovalent Ions

Isovalent and aliovalent substitutions in the cationic and anionic sites of Ca 4 (La 4 Pr 2 )(SiO 4 ) 4 (PO 4 ) 2 O 2 and Ba 4 (La 4 Pr 2 )(SiO 4 ) 4 (PO 4 ) 2 O 2 ceramics were explored in this study. Substitutions of (SiO 4 ) 4− ions with (GeO 4 ) 4− in Ca 4 (La 4 Pr 2 )(SiO 4 ) 4 (PO 4 ) 2 O 2 ceramic increased its densification temperature and thus led to a significant grain growth; however, replacement of Ca 2+ and (PO 4 ) 3− ions by Mg 2+ ions and (VO 4 ) 3− ions, respectively, decreased its densification temperature and resulted in grain refinement. Various rare-earth-metal and (WO 4 ) 2− ion substitutions on La 3+ and Pr 3+ and (PO 4 ) 3− sites, respectively, of Ca 4 (La 4 Pr 2 )(SiO 4 ) 4 (PO 4 ) 2 O 2 ceramic had no impact on the sintering temperature. In the case of Ca 4 (La 4 Pr 2 )(SiO 4 ) 4 (PO 4 ) 2 O 2 apatite with Mg 2+ and (WO 4 ) 2− ion substitutions, second phases were observed in the x-ray diffraction patterns; however, only the pure hexagonal apatite phase with space group P6 3 /m was visible in all the other systems. Overall, Ca 4 (La 2 Nd 2 Pr 2 )(SiO 4 ) 4 (PO 4 ) 2 O 2 ceramic has the best microwave dielectric properties including ε r = 14.2, Q × f = 28,745 GHz and τ f = 0.9 ppm/°C. The densification temperatures of Ba 4 (La 4 Pr 2 )(SiO 4 ) 4 (PO 4 ) 2 O 2 ceramics decreased as the SiO 4 4− and PO 4 3− ions were substituted by WO 4 2− ions. The formation of the secondary phases BaWO 4 and Ba 2 SiO 4 decreased their dielectric constant and increased their Q × f values.